1. Field of the Invention
This invention relates to an electrical circuit interrupting device such as a molded case circuit breaker having a mechanism for applying a force to break open contacts which have become welded closed.
2. Background Information
Certain types of electrical circuit interrupting devices such as molded case circuit breakers, have for each phase a set of separable contacts including in a fixed contact secured to the molded casing and a moveable contact mounted near one end of an elongated contact arm which is pivotally mounted at the opposite end. An operating mechanism actuated manually by a handle, or automatically by a trip mechanism responsive to various over-current conditions, pivots the contact arm to open and close the contacts. Some of these molded case circuit breakers, for instance those disclosed in U.S. Pat. Nos. 4,894,747 and 5,142,112, have a coupling between the operating mechanism and the contact arm which permits the contact arm to blow open through the operation of repulsion forces in response to very large over-currents before the operating mechanism can respond through the trip device. It has been found, however, that high in-rush currents such as those caused by the starting of a motor in a circuit protected by the molded case circuit breaker, can cause the contacts to chatter resulting in welding of the contacts in a closed position which is obviously an undesirable and unsafe condition. Light welds can be broken by the operating mechanism either automatically through operation of the trip device or by application of a manual force in the open direction on the handle. However, the operating mechanism is typically connected to the contact arm close to the pivot point of the arm and, therefore, does not have sufficient mechanical advantage to break stronger welds with the force that can be generated with the operating mechanism.
There is a need, therefore, for an improved circuit interrupting device in which sufficient force can be generated through the operating mechanism to break any welds that may develop between the contacts.
There is a further need for providing such a feature on present circuit breaker designs which does not require extensive modification of the circuit breaker and is not costly.